FirstLight Astronomy Club

33°29.6'N / 117°06.8'W / 1190 ft.

The Not-So-Equal Equinox

There are four important points in earth's orbit around the sun, three of which get a lot of glory, the last of which "falls" by the wayside. They are the solstices and equinoxes.

Three of these guys get the big press. The spring equinox is a traditional "first day of spring." Several high holidays, like Passover and Easter, are embraced around this time. The summer solstice gets credit as the first day of summer and is feted as the "longest day." And winter solstice finds itself fully immersed in tradition and ritual for as far back as we have records. Peoples throughout the northern hemisphere have made that time into a time of great celebration and/or revelry.

Then we have the forlorn, almost unnoticed fall - or autumnal - equinox. We celebrate this event on Monday, but with about as much fanfare as the coming of the new phonebook. So, in our continued attempt to occasionally focus here on the so-called "less important" things, let's take a closer look at this fall equinox.

First, though, we have to understand what on earth a solstice or equinox is. This is going to take into account two of Earth's motions: its orbit around the sun and its tilt. Ready?

You know that our planet revolves around the sun in about 365 days. Moreover, we rotate on an axis once every 24 hours. But the axis of our spin is not perpendicular to the plane of our orbit around the sun - we are tilted over with respect to it. Our spin axis doesn't give a tinker's cuss for the sun; it is lined up with the North Star, Polaris, for as long as we all shall live.

This tilt allows us in the northern hemisphere to sometimes be tilted towards the sun, sometimes away. When we are tilted towards we have the long, hotter days of summer. When tilted away we have shorter days and the weather cools. We are tilted most toward the sun on the summer solstice, most away at the winter solstice.

But it follows that sometime between the days that we are tipped towards the sun and those we are tipped away, there must be a time we are not angled over at all with respect to the sun. These are the equinoxes. We have one of those at the end of March, and one now.

On the equinox the sun will shine on earth from pole to pole. The sun will rise due east, be up for 12 hours, set due west, then be down for twelve. The "daytime," however, will not be really equal to nighttime as the name equinox implies. There are several hi-tech reasons for this inequality which take into account the specifics of true horizon, the size of the sun, and atmospheric refraction, and so on.

But the most obvious reason that daytime wins over nighttime on the equinox is because of twilight, that time before the sun rises and after it sets when the sun still lightens the skies. In Southern California the twilight brackets the "day" for about an extra hour before and after the sun is up.

But at the poles thing are not so ordinary on this day. You might remember from school that on the equinox a person standing at the equator at solar noon will see the sun directly overhead. But put yourself at a pole and you'll see the sun actually going around the horizon like a great blinding ball of light all day long. At the North Pole these are the last days to see the sun for a while as the earth now tilts away from our star causing it to slowly sink below horizon. All this puts the pole in a creepy twilight until we are tilted so far over that from November until January it is nighttime all day long there.

There are precious few holidays on the planet that celebrate this day. Maybe you can start one, one which is filled with thanks for the beautiful fact that our planet is not always lined up perpendicular to the sun. Initiate a holiday that celebrates our whole array of seasons and changing weather and varied climates and maximal living area on this planet, all effects which find their cause in the perfect tilt of our planet.

One for the Little Guys

We are all familiar with the big constellations out there: Orion the Hunter, Ursa Major the Great Bear, Scorpius the... well... Scorpion, and so on. They are monstrous, most of them, stretching across the sky and dotted with intensely bright stars which give them their familiar geometry.

But there are others - actually many others - we have not heard of, much less seen up there. These tiny collections of stars are like bit actors in the Big Scene, filling in the celestial sphere while we focus on the big "stars."

Well, in honor of those disenfranchised constellations, we are going to take a look at four of them which are right above our heads tonight: Vulpecula, Sagitta, Delphinus, and Equuleus. Let's bring them into the spotlight.

We will use a famous asterism to help set our frame of reference, the Summer Triangle.  Go outside in the next few nights during the 9 o'clock hour and face south. Look almost directly above your head. There are three extremely bright stars outlining the great Triangle. The one most southerly, the "bottom" of our triangle, is Altair. And it is in the bottom half of this triangle that we find our first two constellations: Vulpecula and tiny Sagitta.

Vulpecula (Latin for little fox) is a tough one to see in light-polluted skies. It's brightest stars are about 4th magnitude, meaning if you are in a city of blinding lights you will not see the faint fox. In an historical perspective, no one saw the fox until the 17th century when the astronomer Johannes Helvelius put it there. Actually he placed a fox and a goose there, but the fox apparently devoured said goose because it is no longer a part of the constellation. For some reason, only the sly fox remains.

You have a better chance of seeing Sagitta. It is one the smallest constellations in the sky but is accented by two "sort of" bright stars just north of bright Altair, inside our triangle's bottom. Those two stars make up The Arrow, which is what Sagitta means in Latin. Where the arrow comes from is a matter of conjecture. One story has it that the arrow is from nearby Hercules and headed for the eagle, Aquila. Its popularity as an arrow is not just a Graeco-Roman thing, either; the Persians and Hebrews put one there, as well.

Now we leave the triangle. Just to the left of the bottom of the triangle and bordering the Fox and the Arrow is Dephinus, the dolphin. This one you can see on a clear night, and it actually looks rather like a dophin! Go figure.

A little to the east of Altair is the dim quadrangle of stars that make up the dolphin's body, with a star dangling out for its tail. Many cultures place a dolphin or fish there, but the quad itself is also called Job's Coffin. To be sure, the stars look like a coffin, but why "Job's" coffin is a mystery.

And finally, the second tiniest of all the western constellations and one of the most unknown: Equuleus, the Little Horse. Just to the southeast of Delphinus, it is so nondescript I cannot even tell you how to find it other than to refer to the star chart.

That it exists at all is truly a wonder. Why the ancients consecrated just a handful of the dimmest stars in the sky as an official constellation is a puzzle. Is the Little Horse related to giant Pegasus next door? Is it part of some other lesser myth? Why wasn't it just tacked on to a nearby constellation like Delphinus? It could have been The Dolphin in its Froth of Tiny Bubbles.

Learn anything new? Hope so. Anything that may get you money on Jeopardy? Probably not. But if you find yourself a little more interested in the skies above, good for all of us.

Until next time, clear skies!
Temecula Valley High School / Temecula, CA · Some images © Gemini Observatory/AURA Contact Me